1. Field of the Invention
The invention relates to fabrication of graded-index plastic optical fiber.
2. Discussion of the Related Art
Glass optical fiber has become a significant transmission medium in recent years, particularly for long distance transmission applications. Such optical fiber has not found significant usage, however, in smaller scale applications, such as distribution of fiber to the desk in local area networks. In particular, glass optical fiber has not been as cost effective as, for example, copper wire, and also requires extremely precise fiber connections. There has been interest, therefore, in pursuing plastic optical fiber (POF), which offers many of the benefits of glass optical fiber, but is expected to be more cost effective. POF also offers a larger core, which makes connection and splicing easier.
Initially, step index POF (having a core of one refractive index, surrounded by a cladding of a different refractive index) was manufactured and used. Unfortunately, the modes traveling along a step index fiber experienced an undesirably high level of dispersion, thereby limiting the fiber's capability. In response to this problem, graded-index POF (GI-POF) was developed, which possesses a varying refractive index from the core to the cladding layer. GI-POF exhibits a lower level of mode dispersion, thereby providing improved properties. GI-POF, however, was more difficult, and thus more expensive, to manufacture than step-index POF. Improved methods for manufacturing GI-POF were therefore sought.
One method of forming GI-POF is to start with a preform, similar to the preform from which glass optical fiber is generally drawn. See, e.g., U.S. Pat. Nos. 5,639,512 and 5,614,253, which discuss a process for chemical vapor deposition (CVD) formation of a preform for GI-POF. According to the process, a polymer and a refractive index modifier are deposited onto a rod, and the amount of refractive index modifier is varied during the deposition to provide the desired refractive index profile. While such preforms are useful for preparing GI-POF, easier processes are desired.
One alternative to preform formation is extrusion, which is commonly used with plastics to form a variety of items. Extrusion was expected to be quicker and cheaper than forming and drawing a preform, but the need for a graded refractive index profile created complications. U.S. Pat. No. 5,593,621 (the '621 patent) discusses an extrusion process for GI-POF. According to the '621 patent, GI-POF is manufactured by extruding one material circumferentially around another material, e.g., by use of a concentric nozzle. At least one of the materials contains a diffusible material having a distinct refractive index, such that the diffusion of the material provides the desired refractive index contrast. The method of the '621 patent appears to offer a functional process, but also appears to exhibit several drawbacks.
In particular, it is not clear that the process is able to be performed without providing a delay time (stopping the flow of material) or a very slow extrusion speed, to allow the diffusible material sufficient time to diffuse. Specifically, the examples disclose a small distance, 3 cm, between the outlet of concentric nozzle 5 (see FIG. 1) and the outlet of core nozzle 3. Thus, the two materials are in contact only over this small distance before exiting the apparatus. It is unclear whether this small contact distance allows sufficient diffusion, without requiring either intermittent stoppage or an extremely slow extrusion speed. It appears that either stoppage or low speed was used, because, for example, Embodiment 6 states that diffusion was effected for about 3 minutes within this contact region, and Embodiments 7, 8, and 9 all state that diffusion occurred for about 10 minutes in the contact region. Unfortunately, the reference does not disclose an extrusion speed nor make clear whether the process had to be halted intermittently. In addition, there is no information on how to predict the refractive index profile in the resulting fiber, and trial-and-error is apparently required to find appropriate process parameters.
Neither intermittent stoppage nor extremely slow extrusion speed is attractive from a commercial standpoint. Intermittent stoppage slows the process and creates discontinuities in the refractive index profile of the fiber. And a slow extrusion speed increases both the cost of the process and the time involved. Thus, improvements in processes for extruding graded index plastic optical fiber are desired. It would also be desirable to predict the refractive index profile that would result from a particular extrusion process, such that burdensome trial-and-error could be avoided.